1. Field of the Invention
This invention relates to lasers and more particularly to a tunable multiline/multiband laser resonator.
2. Description of Related Art
Laser systems based primarily on molecular gases have many discrete vibrational rotational transitions upon which laser action can be made to occur. While CO.sub.2 lasers will be used throughout, those skilled in the art will recognize the concepts are applicable to other gases (CO, NH3), excimers, solid state materials and organic dyes.
In CO.sub.2, the upper and lower laser levels responsible for the 10.6 micrometer band emission are the (001) and the (100) vibrational states, respectively, each of which have many rotational components. Lasing occurs from a single rotational component J' of (100) according to the Quantum Mechanical selection rules: EQU dJ=+1,-1,0
and EQU J'=0 .rarw..parallel..fwdarw. J"=0
In a broad band resonator, a single transition will dominate generally over all others, the transition for which there is the highest gain [P(20) of the 10.6 micron band]. As the rotational levels are tightly coupled, the levels not participating in the laser action redistribute themselves in an attempt to maintain a Boltzman like distribution as the population of J=19 is reduced by stimulated emission. If during lasing equilibrium is established, a large fraction of the energy stored in all of the high J value rotational states of the upper vibrational level will be effectively funneled into the single lasing rotational level. The relaxation of the rotational levels is in part responsible for the high efficiency of the CO.sub.2 laser.
For proper laser target interaction, it was believed that for CO.sub.2 fusion lasers nanosecond pulses were necessary. Q-switched TEA lasers were capable of such pulse lengths but inefficient. The (001) relaxation rates were not fast enough to couple any significant amount of energy from the rotational manifold to a single rotational level on such a short time scale [G. T. Schappert, Appl. Phys. Lett., 23, 319, (1973); B. J. Feldman, IEEE J. Quantum Electron, QE-9, 1070 (1973): K. Smith and R. M. Thomson, Computer Modeling of Gas Lasers, Plenum Press, New York, 1978, pp. 136-154.: E. P. Velikhov, Molecular Gas Lasers, MIR Publishers, Moscow, 1981, pp 214-225.] Then it was found that more efficient operation occurred when output from several transitions simultaneously was encouraged and the multiband oscillator at the 10.6 micron band was developed [H. Baumbacker and R. S. Long, Phys. Lett., 47A, 429 (1974)] and followed by the development of simultaneous emission at the 10.6 micron and 9.4 micron bands [J. F. Figueria and H. D. Sutphin, Appl. Phys. Lett., 25, 661 (1974)]. Although the total pulse energy for these multiling oscillators waws increased, the output on any given line ase decreased compared to single frequency emission.
A master and slave oscillator array was developed for a directed energy weapon; this system though suitable for preselected v-J multiline oscillation is too costly and inefficient for use in smaller inexpensive resonators and impractical for use on many v-J transitions.
In other applications, such as, for example, remote chemical detection, the laser is required to transmit a variety of wavelengths corresponding to absorption bands of the chemical species which is to the detected. If the nature of the chemical species is unknown prior to the measurement, then the laser must be scanned across a large number of output wavelengths to identify the absorption signature of the unknown species.
Presently, in differential absorption LIDAR (DIAL) applications a single laser is scanned as rapidly as possible, and the output is seen as a series of pulses at different wavelengths. To increase the data collection rates, multiple lasers are incorporated into a single device which would then be capable of rapid multiple wavelength emission. [J. L. Bufton, T. Itabe, and D. A. Grolemund, "Airborne Remote Sensing Measurements with a Pulsed CO.sub.2 Dial System", Optical and Laser Remote Sensing, Springer Verlag, New York, 1983]. A disadvantage of such system is the requirement for independent power supplies, pulse forming networks, switching elements and resonators. Also, sophisticated circuitry is required to control the wavelength of each laser and to ensure the proper sequential firing of the independent lasers. Further disadvantages are the complexity, size and weight, and cost of the system.